Vacuum fluorescent displays (VFDs) are typically manufactured in the form of a glass housing having an evacuated central cavity containing phosphored anode segments arranged in a suitable display pattern, a filament (cathode), and a grid disposed between the filament and the anode segments. The filament is electrically heated to generate a cloud of electrons, and selected anode segments are illuminated when positive voltages are supplied to the grid and the selected anode segments. And to turn off the display, the grid is held at a negative potential with respect to the filament.
In applications where a single DC voltage source is used to drive a VFD, one end of the filament can be coupled to the source voltage, with the other end of the filament being coupled to ground potential (i.e., the negative terminal of the voltage source) through a voltage-dropping device that establishes a cut-off voltage for the display. In this way, the display may be turned off without having to provide a grid voltage that is negative with respect to ground potential. Typically, the voltage-dropping device is mechanized with a resistor or a string of series-connected forward-biased diodes, for example. The anode/grid drive circuit and other electrical loads associated with the display are powered by the voltage source through separate power supply circuits.
The main problem with the above-described arrangement is that the power dissipated in the voltage-dropping device is essentially wasted. Additionally, separately supplying power to the filament and the other electrical loads is not particularly cost-effective. Accordingly, what is needed is a more efficient and cost-effective drive arrangement for a VFD.